Factory


1   /*******************************************************************************
2    * Copyright (c) 2002, 2005 Object Factory Inc.
3    * All rights reserved. This program and the accompanying materials 
4    * are made available under the terms of the Eclipse Public License v1.0
5    * which accompanies this distribution, and is available at
6    * http://www.eclipse.org/legal/epl-v10.html
7    * 
8    * Contributors:
9    *      Object Factory Inc. - Initial implementation
10   *******************************************************************************/
11  package org.eclipse.ant.internal.ui.dtd.util;
12  
13  import java.lang.ref.SoftReference  ;
14  
15  /**
16   * Factory maintains a free list and, with FactoryObject,
17   * serves as a basis for factories of all types.
18   * Factory should only be subclassed in singleton classes;
19   * for static factories, it may be instantiated as
20   * a static object.
21   * @author Bob Foster
22   */
23  public class Factory {
24      
25      /**
26       * Return the first object on the free list
27       * or null if none.
28       */
29      public FactoryObject getFree() {
30          Head head = getHead();
31          FactoryObject obj = head.next;
32          if (obj != null) {
33              head.next = obj.next();
34              obj.next(null);
35          }
36          return obj;
37      }
38      
39      /**
40       * Add an object to the free list.
41       */
42      public void setFree(FactoryObject obj) {
43          Head head = getHead();
44          obj.next(head.next);
45          head.next = obj;
46      }
47      
48      private Head getHead() {
49          Head head = (Head) free.get();
50          if (head == null) {
51              // head is needed because you can't change
52              // the referent of a SoftReference.
53              // Without head, we would need to create
54              // a new SoftReference each time we remove
55              // a map from the list. With head, getting
56              // a free object only causes memory allocation
57              // when the list has been previously collected.
58              head = new Head();
59              free = new SoftReference  (head);
60          }
61          return head;
62      }
63  
64      private static class Head {
65          public FactoryObject next;
66      }
67      private SoftReference   free = new SoftReference  (new Head());
68  }
69
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